Medical practitioners often need to invasively measure fluid pressure within a body. Commonly, blood pressure, muscle compartment pressure, and spinal column pressure are taken to assist in the diagnosis or treatment of patients. For those with spinal problems, a medical practitioner often performs a procedure, commonly called a spinal tap, to invasively enter the spinal column, withdraw some spinal fluid, and measure the pressure within the spinal column. The spinal tap is more formally called the Lumbar Puncture Procedure (LPP).
The technology for the Lumbar Puncture Procedure has changed little since it was first performed by Quincke in 1891. This procedure removes Cerebrospinal Fluid (CSF) from within the spinal column for bacteriologic evaluation and measures the CSF pressure to diagnose the patient's general neurologic condition. Unusually high or low CSF pressures indicate some loss of the patients auto-regulation (self-regulating capacity) and general loss of neurologic function.
The current invasive implement used in LPP is a small needle cannula. An obturator or stilette is positioned inside the needle to prevent matter from blocking the internal channel of the needle as it penetrates tissue. The needle is inserted between the lumbar vertebrae just below the termination of the spinal cord, piercing the dura and reaching the fluid filled sub-dural space in the spinal column. Once located in this space, the obturator or stilette is removed, allowing CSF to flow from the needle tip and out the proximate end of the needle. The proper location of the needle is ascertained by this outflow of CSF.
A stopcock and open tube manometer now may be connected to the proximal end of the needle cannula. With the stopcock in the open position, the manometer fills with CSF until its elevation (Pressure Head) equals the pressure inside the spinal column. Equalizing the pressure can take between 5 and 15 minutes. This height is converted to a pressure unit (e.g. mmHg or cmH.sub.2 O) and is defined as the patient's "Opening Pressure". After this pressure measurement, some of the fluid contents of the manometer can then be removed through a release valve into a collection vial for bacteriologic evaluation, such as diagnosis for spinal meningitis.
Once a sufficient quantity of CSF has been removed for analysis (typically 9 to 15 ml) the manometer can be re-filled as above (an additional 5 to 15 minutes). Once stabilized this new pressure measurement on the manometer is said to be the patients "Closing Pressure". If the difference between the Opening and Closing pressures is large, this may indicate a spinal blockage that prevents the normal auto-regulation function that should compensate for the loss of fluid and maintain a constant CSF pressure. Many physicians skip this "Closing Pressure" portion of the procedure to save time even though it is important in assessing the patients condition.
If a blockage is diagnosed by a large drop in CSF pressure, then the removal of an excessive amount of CSF may cause patient harm: if too much fluid is removed the pressure gradients between the brain and spinal column may cause traction and damage to the brain stem. In a blockage case, it would be preferable that the pressure measurement be made in a manner minimizing the removal of CSF. In fact, if the patient's CSF pressure is believed to be too high this procedure is contraindicated.
Also, using the LPP method takes considerable time and effort. The longer the needle cannula is left in the patient, the more likely an infection will result. Additionally, the extended procedure time contributes to a common post operative complication called "Post Lumbar Puncture Headache". One of the primary causes of the complication is the continued leakage of CSF from the puncture site. This causes continued traction on the spinal cord and brain stem, resulting in patient headaches. As a result, the patient must remain hospitalized in a prone position for days, adding to the cost and discomfort of these patients.
The current manometer systems are typically over 18 inches tall. As a result practitioners have a difficult time holding the manometer/needle system in a stable position for the 10-20 minute duration of the procedure. This "Level Arm" effect contributes to additional tearing of the tissues that increase the amount of CSF leakage.
There have been attempts to use alternative detection systems rather than manometer systems in the measuring of body fluid pressures, such as Tempkin et al., U.S. Pat. No. 3,610,228, Moriuchi et al., U.S. Pat. No. 4,790,193, and Davis et al., U.S. Pat. No. 4,817,629. Each of these referenced devices requires either a complex valve switching system, a chamber that must be filled with a fluid, or an external device for priming, with corresponding limitations on usefulness.